Packaging containers of the single use disposable type for liquid foods are often produced from a packaging laminate based on paperboard or carton. One such commonly occurring packaging container is marketed under the trademark Tetra Brik Aseptic® and is principally employed for aseptic packaging of liquid foods such as milk, fruit juices etc, sold for long term ambient storage. The packaging material in this known packaging container is typically a laminate comprising a bulk core layer of paper or paperboard and outer, liquid-tight layers of thermoplastics. In order to render the packaging container gas-tight, in particular oxygen gas-tight, for example for the purpose of aseptic packaging and packaging of milk or fruit juice, the laminate in these packaging containers normally comprises at least one additional layer, most commonly an aluminium foil, which moreover renders the packaging material thermosealable by inductive thermosealing which is a rapid and efficient sealing technique for obtaining mechanically strong, liquid- and gas-tight sealing joints or seams during the production of the containers.
On the inside of the laminate, i.e. the side intended to face the filled food contents of a container produced from the laminate, there is an innermost thermoplastic heat sealing layer, applied onto the aluminium foil, which innermost, inside layer may be composed of one or several part layers, comprising heat sealable adhesive polymers and/or polyolefins. Most commonly, the thermoplastic heat sealable polymer is a low density polyethylene, which normally has adequate properties for heat sealing and for the function of moisture and liquid barrier towards the filled contents of the package. Also on the outside of the core layer, there is an outermost heat sealable polymer layer.
By innermost or inside layer is meant a layer which is applied on the side of the packaging laminate facing towards the inside of a packaging container formed from the laminate, and which will be in contact with the filled contents of a filled packaging container.
The packaging containers are generally produced by means of modern, high-speed packaging machines of the type that form, fill and seal packages from a web or from prefabricated blanks of packaging material. Packaging containers may thus be produced by reforming a web of the laminated packaging material into a tube by both of the longitudinal edges of the web being united to each other in an overlap joint by welding together the inner- and outermost heat sealable thermoplastic polymer layers. The tube is filled with the intended liquid food product and is thereafter divided into individual packages by repeated transversal seals of the tube at a predetermined distance from each other below the level of the contents in the tube. The packages are separated from the tube by incisions along the transversal seals and are given the desired geometric configuration, normally parallelepipedic, by fold formation along prepared crease lines in the packaging material.
The main advantage of this continuous tube-forming, filling and sealing packaging method concept is that the web may be sterilised continuously just before tube-forming, thus providing for the possibility of an aseptic packaging method, i.e. a method wherein the liquid content to be filled as well as the packaging material itself are reduced from bacteria and the filled packaging container is produced under clean circumstances such that the filled package may be stored for a long time even at ambient temperature, without the risk of growth of micro-organisms in the filled product. Another important advantage of the Tetra Brik®-type packaging method is, as stated above, the possibility of continuous high-speed packaging, which has considerable impact on cost efficiency.
From the consumer's point of view, it is desirable that the packaging container be easy to handle and easy to open when it is time to empty the package of its contents, and in order to satisfy this need, the packaging container is often provided with some type of opening arrangement, with the aid of which it may readily be opened without the need to employ scissors or other implements.
A commonly occurring opening arrangement in such packaging containers includes a hole punched in the core layer of the package wall, the hole being covered on the inside and outside of the package wall, by the respective outermost and innermost layers of the packaging wall which are sealed to one another in the region of the opening contour of the through-going hole, thus forming a membrane of the layers not being paperboard. One example of a prior art opening arrangement has a separate pull-tab or opening strip which is applied over the hole and which is rupturably sealed to the outer layer of the outside of the package wall along a sealing joint around the entire opening contour of the hole and at the same time permanently sealed to the outer layer in the region inside the opening contour of the hole.
In more advanced opening arrangements, an opening device, usually of moulded plastics, having a pouring spout and a screw top for resealing, is applied onto the region of and around the hole, which opening device is designed to penetrate or remove the membrane within the hole region by a pushing-down or screwing-down movement or, alternatively, to remove the membrane by a screwing- and/or pulling-up movement of the opening device. In the latter kind of opening device, the inside of a screwable part of the opening device is adhered to the membrane of the hole, in such a way that when it is screwed upwards away from the packaging wall, the membrane is lifted along with the screwable part and torn away from the edges of the hole, leaving a practically clean-cut hole for pouring the filled contents out of the package.
Further examples of more advanced opening arrangement are those with a cutting function, i.e. having a protruding part, such as a sharp edge or a peak that is forced to cut through the material within the hole membrane when the opening cork is screwed to open.
A precondition for any such an opening arrangement to function efficiently and expediently is that, there is adequate adhesion between the different layers of the membrane such that it does not delaminate when screwing- and pulling-down/pushing-up forces or cutting forces are applied to it during the opening operation.
It is generally difficult to obtain such adequate adhesion within the regions of the holes, because of the difference in total laminate thickness between the regions of the holes and the regions outside of the holes when laminating together the aluminium foil and thermoplastic layers of the membrane. When passing a web of the laminated layers through a press nip in a lamination station, the layers are pressed to adhere to each other by means of a pressure roller and a cooled cylinder. In the regions defined by the hole or the slit, the press nip is unable to press the layers together sufficiently for achieving the requisite adherence.
Thus, the thickness variations of the core layer may cause that the barrier layer, usually an aluminium foil, which is relatively thin, will not be pressed against and adhered sufficiently well to the surrounding layers of thermoplastics within the whole of the region defined by the hole, which means that air may be entrapped adjacent to the edges of the holes. Also, because the materials of the layers within the membrane laminate are not stable to heat load in the lamination steps, i.e. the layers are not sufficiently thermomechanically stable to keep their dimensions when heated up, there will be ruptures, cracks and other damages in the layers of the membrane.
This means that there may be fracture formations or cracks in the barrier layer, which may lead to the gas-tightness of the packaging container being impaired. Consequently, the colour, taste and nutritional values of the packaged food product may be deteriorated. Furthermore, the integrity of the package may be impaired.
The air inclusions also result in it being difficult to tear off or penetrate the membrane consisting of the barrier layer and the polymeric films in the hole or slit, with the ability to open the packaging being restricted and/or with it not being possible to make a clean cut when penetrating, resulting in the formation of frayed edges.
All these problems become more accentuated as the speed of the conversion from raw materials into a laminated packaging material web, is increased.